Inching is the process by which an operator controls the slow forward or reverse travel movement of a motorized vehicle by the light application of clutch torque through the transmission. Certain industrial vehicles including materials handling vehicles or fork lift trucks, include a dual-purpose inch/brake pedal. The inch/brake pedal operates to engage a vehicle braking system, and also to engage a vehicle transmission. Typically the braking system is fully engaged when the inch/brake pedal is fully depressed, whereas the vehicle transmission is fully engaged when the inch/brake pedal is fully released. Inching occurs in an intermediate range of motion of the inch/brake pedal when the vehicle transmission is only partially engaged. Many industrial vehicles further include an accelerator pedal which is used to control the engine speed. The accelerator pedal has no effect on the clutch torque until the vehicle transmission is engaged.
The present state-of-the-art is to provide a fixed or manually adjustable amount of overlap of transmission drive torque to service brake torque according to the position of the inching/brake pedal alone or in combination with the service brake torque or brake pressure. This is known as inch/brake overlap. For vehicles that provide for adjustable overlap, a service technician manually adjusts the amount of overlap while the vehicle is being serviced. The amount of overlap is accordingly fixed at the adjusted amount during subsequent operation of the vehicle, until the overlap is once again manually adjusted by a service technician.
As operating conditions of the vehicle change from one operating shift to the next, or indeed during the same operating shift, the fixed amount of inch/brake overlap is well suited for some operations and not for others. For example, if the inch/brake overlap is manually set at a low value, this works well when the vehicle is operating on level surfaces. The operator is able to control vehicle inching satisfactorily under normal conditions. However this same low value does not work well when the vehicle is operating on an inclined surface, in which case the vehicle will roll down the hill when the brake pressure decreases too low without sufficient transmission force to maintain a position of the vehicle on, or to move the vehicle up, the grade. During loading or unloading operations on an incline, unintended vehicle movement down the grade may cause damage to the load being moved, or to other equipment or vehicles located adjacent the load.
If the inch/brake overlap is manually set at a high level for vehicle operations on an inclined surface, this will improve the hill holding operation of the vehicle. However, the high level of inch/brake overlap will result in an unnecessary buildup of heat in the transmission and braking systems as they work against each other. This results in more frequent and expensive vehicle maintenance requirements, and is undesirable when the primary application of the vehicle is on a level operating surface. Inching operation of the vehicle when the inch/brake overlap is high also affects the degree of fine controllability of the vehicle, tending to cause the vehicle to lurch or operate unevenly.
The present invention addresses these and other problems.